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Bureau of Mines Information Circular/1986 



Cutter Roof Failure: An Overview 
of the Causes and l\/lethods for 
Control 



By John L. Hill III 



UNITED STATES DEPARTMENT OF THE INTERIOR 



Information Circular 9094 



Cutter Roof Failure: An Overview 
of the Causes and IVIethods for 
Control 



By John L. Hill III 




UNITED STATES DEPARTMENT OF THE INTERIOR 
Donald Paul Hodel, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 







Library of Congress Cataloging in Publication Data 



Hill, John L. 

Cutter roof failure: An overview of the causes and methods for control. 



Ilnformation circular / United States Department of the Interior, Bureau of Mines; 9094 i 

Bibliography: p, 26 

Supt. of Docs, no.: I 28.27: 

1. Mine roof control. 2. Rock deformation. 
I. Title. II. Series: Information circular (United States. Bureau of Mines 1; 9094 

-^FN2a5J[J4_ ITN2881 622 s |622'.26| 86-600247 



CONTENTS 

Page Page 

Abstract 1 Using prediction of failure in mine design 19 

Introduction 2 Selecting control measures 19 

Background 4 Artificial support 19 

Factors in the propagation of cutter roof failure . . 5 Angle bolting 20 

Openings in a rock mass 5 Truss bolting 21 

Single openings 6 Other supports 22 

Multiple openings 7 Mine design changes 22 

Stress environment 7 Sacrifice entries 22 

Regional stresses 8 Pillar softening and yield pillars 23 

Stress concentrations beneath stream valleys . 9 Roof slotting 24 

Rock mass characteristics 13 Entry reorientation 25 

Rock strength and stiffness 13 Indirect control measures 26 

Minor geologic structures 14 Conclusions and recommendations 26 

References 26 

ILLUSTRATIONS 

1. Cutter roof failure sequence 2 

2. Initial propagation of cutter roof failure to weak bedding plane 3 

3. Severe cutter roof failure 3 

4. Massive roof fail 4 

5. Stress concentration diagrams of rectangular openings with width-to-height ratios of 1, 2, and 3 under 

gravitational loading conditions 6 

6. Stress concentration diagram of six adjacent openings under gravitational loading conditions 7 

7. Stress concentration diagrams of rectangular openings with width-to-height ratio of 3 and loading 

conditions of horizontal-to-vertical-stress ratios of 1, 2, and 3 8 

8. Qualitative example of influence of horizontal-to-vertical-stress ratio on angle of failure propagation 8 

9. Analysis of elemental components of angle of failure as related to stress environment and rock properties . 8 

10. Horizontal in situ compressive stress measurements in the United States 9 

11. Partial mine map from western Pennsylvania, showing preferential orientation of roof falls 10 

12. Mine map from mine in southern West Virginia, showing correlation of roof fall locations with centers of 

overlying stream valleys 11 

13. Values of stress for critical points of an opening beneath a valley versus an opening beneath a hill, for an 

increasing height of the hill 12 

14. Stress concentration in roof-rib corner versus elasticity of coalbed 13 

15. Stress values in roof-rib corner and midspan of roof versus ratio of thickness of the two immediate roof 

members 14 

16. Qualitative interpretation of cutter failure propagation in thinly bedded, single roof rock type 14 

17. Clastic dike in coal pillar 15 

18. Clastic dike in midspan of roof rock of crosscut 16 

19. Geologic and roof failure map of Main A of Greenwich North Mine 17 

20. Clastic dike in-mine occurrences in Eastern United States 17 

21. Section of mine in southern West Virginia, showing location of coalbed roll and local roof falls 18 

22. Cross section of roll A-A' from figure 21 18 

23. Stress values for critical points of an entry versus severity of pitch of an overlying roll 19 

24. Decision process diagram for determining the cause of cutter roof failure and selecting control measures . . 20 

25. Angle bolting hardware 21 

26. Angle bolt installation 21 

27. Two basic designs of roof bolt trusses 21 

28. Plan view of truss installation 22 

29. Plan view of mine entry showing placement of cribbing adjacent to clastic dike to deter cutter failure 

formation 23 

30. Contemporary versions of caving chambers for use in three-entry gateroad configurations 23 

31. Plan for placement of auger holes for pillar-softening concept 23 

32. Cross-sectional view of rib-slotting method 24 

33. Placement of holes for roof-slotting method 24 

34. Apparent values of horizontal in situ stress for the two extremes of entry orientation versus orientation of 

actual principal in situ stress 25 

TABLES 

1 . Maximum shear stress values and analysis of failure 9 

'I Horizontal in situ stress measurements 10 



UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 



deg 


degree 


ft 


foot 


in 


inch 


Ib/ft^ 


pound per square foot 



Ib/ft^ 


pound per cubic foot 


pet 


percent 


psi 


pound per square inch 



CUTTER ROOF FAILURE: 
AN OVERVIEW OF THE CAUSES AND METHODS FOR CONTROL 



By John L. Hill IIP 



ABSTRACT 

The Bureau of Mines is conducting research on the causes and methods for control 
of cutter roof failure in underground coal mines. This hazardous ground control 
problem exposes miners to the danger of falling roof rock and frequently results in 
massive roof failure. This report outlines the probable causes of cutter roof failure, 
which are proposed based on field investigations, numerical model analysis, and 
in-mine observations. Traditional methods of control are presented, as well as 
innovative methods based on mining concepts developed during earlier years of coal 
mining history. The report can be useful to a mine operator for assessing the causes of 
cutter roof failure on a site-by-site basis and for predicting the probability of its 
occurrence. A process is presented for selecting an optimum control method that 
includes both traditional and innovative control techniques for each of the various 
causes of cutter roof failure. 



'Geologist, Pittsburgh Research Center. Bureau of Mines, Pittsburgh, PA. 



INTRODUCTION 



Ground control research conducted by the Bureau of 
Mines is designed to develop technology that will aid in 
reducing the frequency of accidents associated with poor 
ground control conditions. Cutter roof failure often poses a 
safety hazard to miners and causes delays in production 
while massive roof falls are cleaned up and unstable roof 
is resupported. Implementation of control measures 
reduces the threat of injury to miners and prevents 
production delays. 

The definition of cutter roof failure used in this report 
separates this unique type of failure from other ground 
control problems, to aid in the analysis of the causes and 
in the development of control methods. The definition is as 
follows: 

Cutter roof failure in mine roof rock is a failure 
process that initially begins as a fracture plane in the roof 
rock parallel to, and located at, the roof-rib intersection. 
The fracture propagates upward into the roof over the 
mine opening at an angle usually steeper than 60° from 
the horizontal. 

The mappable extent of cutter failure may range in 
length from only a few feet to several hundred feet and 
may traverse entry-crosscut intersections without change 
in direction. Once initiated along the roof-rib line, a cutter 
may propagate away from the roof-rib line of one side of a 
room, cross the roof span, and continue along the other 
side of the room. It is important to note that this definition 
excludes a similar type of failure (sometimes referred to as 
"kink roof) that has comparable characteristics but 
occurs in the center of the entry. (The reasons for this 
distinction are discussed in the "Background" section.) 
Researchers refer to the cutter failure discussed in this 
report as "classic" cutter roof failure; except for short 
deviations across intersections and entries, initial failure 
is confined to the roof-rib intersection. 

The cutter failure sequence is illustrated in figure 1. 
An in-mine example of figures lA and IB can be seen in 
figure 2, which shows roof rock in a crosscut as viewed 
from an intersecting entry. The initial vertical fracture 
propagated from the roof-rib line until it reached a weak 
bedding plane. At that point, cribbing was installed to 
prevent collapse. Figure 3 is another example of cutter 
failure as it may appear when the vertical fracture 
propagates to a much higher level in the roof If adequate 
support is not installed and failure is allowed to progress, 
massive roof failure will ensue, as shown in figure 4, 

Cutter roof failure has long been known to be most 
prevalent in the Appalachian Coalfields. However, 
ongoing reconnaissance research by the Bureau is 
revealing that cutter roof failure occurs in each of the 
major coal basins of the United States where underground 
mining is practiced. In each case, its occurrence may 
appear to be unpredictable. An entire mine may be 
experiencing ideal ground control conditions when sud- 
denly a working section encounters this failure with 
seemingly no explanation. For other mines, cutter failure 
is a chronic condition, and when conditions become only 




-7 Roof bolts^ 



V - Direction of cutter 
\ (fracture) propagation 

^' ---Nearly vertical crack 
^ (fracture)^^ . 



Entry or crosscut 



Underclay 




A INITIAL CUTTER FAILURE 



Fracture propagating across 
the entry or crosscut at 
anchorage horizon (bed 
separation occurs at this 
weak zone) 




B, PROPAGATION OF CUTTER ALONG WEAK ZONE 




Undercloy 



C, CUTTER FALL 

Figure 1. — Cutter roof failure sequence. Not to scale. 



slightly worse than normal, entire working sections may 
have to be abandoned, resulting in the sterilization of 
large tracts of coal reserves. In either case, control of 
cutter roof failure is no easy task. Simply changing bolt 
length or placing a crib in a strategic location may have no 
effect at all. Thus, based on the nature of cutter roof 
failure, and the problems with trying to control its 
occurrence. Three components of the issue become evident 
and are the subject of this report: (1) causes, (2) prediction, 
and (3) control. 




Figure 2.— Initial propagation of cutter roof failure to a weak bedding plane In roof rock above a crosscut. 




Figure 3. — Severe cutter roof failure supported by fiber cribbing. 




Figure 4. — Massive roof fall resulting from unsupported cutter roof failure. 



BACKGROUND 



Theoretical explanations of the causes of cutter roof 
failure exist; however, in-mine verification of these 
theories through instrumentation and mapping have been 
far from comprehensive. The publications discussed in 
this section are representative of the present understand- 
ing of cutter roof failure and applicable control measures, 
and each of the concepts discussed here will be developed 
further in following sections. 

In 1948, R. W. Roley {43Y published an article with 
reference to a particular type of roof failure, which he 
referred to as "pressure-cutting." Many of the charateris- 
tics that Roley described are the same ones used to 
describe the current term "cutter roof failure." Although 
cutter roof failure has been found to be most prevalent in 
the Appalachian Coalfields, Roley originally described 
this type of failure in the Illinois Basin. For an 
explanation of its occurrence, he borrowed from D. W. 
Phillips {41 ), who cited abnormally high lateral pressures 
or stress conditions, of regional magnitude, causing shear 
forces in the roof rock at the rib line to exceed the shear 
strength of the rock. 

Rock type as an important factor in the development 
of cutter roof failure was discussed by Thomas (45) in 
1950. He observed that the immediate roof rock type must 
be competent with respect to the overlying roof rock; 
otherwise, failure will occur in the center of the entry. In 
addition, Thomas promoted the use of roof bolting as 



^Italicized numbers in parentheses refer to items in the list of references 
at the end of this report. 



opposed to timbering for successful support of the roof, and 
proposed angle bolting as an effective control measure, 
borrowed from the lead and zinc mining industry (46). 
Thomas identified rock tjrpe as a cause for localized 
occurrences of cutter roof failure. For local occurrences 
where no change in roof rock type was found, a possible 
cause was suggested in 1961 when Lang (28) demon- 
strated that stress concentrations exist beneath stream 
valleys creating an unstable environment for mining. 

A first step toward assessing the influence of high 
lateral stress and roof rock competency on the formation of 
cutter roof failure was taken by Wang (47-48), using 
two-dimensional, finite-element analysis. His results 
mirrored observations that had been made in the field; 
with this correlation, it was suspected that regionally 
high horizontal stress and stress induced by overlying 
stream valleys and overlying structural features such as 
paleochennels were causing failure. With the aid of 
computer modeling, he analyzed the effect of pillar 
strength on the distribution of stress in the upper corners 
of the entry, with a result that pointed at a possible 
method of control. Wang found that by weakening the 
pillar to some specific depth from the pillar skin, shear 
stresses in these critical areas were reduced and the 
threat of cutter failure could be lessened. A limited 
amount of in-mine testing produced inconclusive results 
(34). 

In-mine verification of regionally high lateral stress 
was necessary to determine its influence on the propaga- 
tion of cutter roof failure. Agapito (IT and Aggson (2-4) 



both reported on an investigation conducted in several 
mines of southern West Virginia. The mines were located 
in the Beckley Coalbed, and overcoring techniques were 
employed to determine the horizontal in situ stress. The 
measurements were found to be much greater in 
magnitude than would be expected if the horizontal stress 
was attributable only to the Poisson ratio effect of the 
overburden. A two-dimensional, finite-element model 
representative of the minesites was analyzed, and a close 
correlation was found between in-mine observations of 
cutter roof failure and the failure modeled by the 
computer. Three important findings were made through 
this method of in-mine verification: 

1. The horizontal stress field in the region was found 
to be relatively uniform in magnitude and orientation. 

2. The angle of the failure surface with the horizontal 
is dependent, at least partly, upon the relative magni- 
tudes of the horizontal and vertical stresses. 

3. Pillars designed to completely yield reduce roof 
stresses by 15 pet. 

Each of these significant findings is elaborated upon later 
in the text. 

Kripakov (27) conducted a similar study. Using 
in-mine, in situ stress measurements and two- 
dimensional, finite-element analysis, he assessed the 
applicability of Wang's pillar-softening concepts (34, 47). 
According to the model, it was found that, for the 
particular stress state of the mine, the pillar-softening 
concept showed promise, provided certain modifications 
were made. Kripakov (27) recommended that the longwall 
method of mining be used in areas of severe cutter roof 
failure, which would allow for less exposed roof area in 



need of support and for the possible use of sacrifice entries 
on advance of panels. His recommendations also included 
continued use of truss bolting, which has also been 
recommended by others (5, 19, 23, 29-30). 

These in-mine investigations and subsequent analy- 
ses accounted for only the immediate rock properties and 
minor rock structure such as bedding. Although Wang 
conducted computer analysis of the influence of 
paleochannels, in-mine stress analysis of these features 
has not been conducted. The influenc^e of geologic 
structures, such as the presence of clastic dikes, (clay 
veins), has recently been explored by lannacchione (23) 
and Hill (19). Through detailed in-mine geologic mapping, 
these two independent studies found that the presence of 
clastic dikes contributed to the instability of roof rock, 
resulting in the initiation of cutter roof failure. Although 
only three mines were considered in these investigations, 
it is highly probable that clastic dikes influence the 
formation of cutter failure in many other mines, and 
geology should certainly not be ignored. The contribution 
of clastic dikes to the failure mechanism is discussed in 
the section "Minor Geologic Structures." 

With respect to current research in cutter roof failure, 
mine operators are reemploying techniques of sacrifice 
entries, yielding pillars, and reorientation of headings in 
an attempt to solve their cutter failure problems. This 
trend in the industry toward alternative control measures 
shows promise for success. The following explanations for 
the causes of cutter failure and methods for control 
provide a systematic approach to aid the operator in 
determining which technique is best suited to a particular 
situation. 



FACTORS IN THE PROPAGATION OF CUTTER ROOF FAILURE 



Prior to applying various control measures to a 
particular ground control problem, the mechanisms 
responsible for its initiation and subsequent propagation 
should be understood. The problem of cutter roof failure is 
an excellent example of this, because many trial and error 
methods have been unsuccessful in controlling it. Two 
interdependent variables, cited in the preceding section, 
are critical factors in the formation of cutter roof failure: 
the stress environment and rock type. The stress 
environment is further influenced by the addition of 
adjacent entries, the introduction of forward and lateral 
abutment pressures induced by retreat mining, and the 
stresses created by the simultaneous extraction of 
multiple seams. An additional variable, equally impor- 
tant, is that of opening dimensions, which directly affect 
the distribution of stress concentrations around the 
periphery of the opening. 

To help the reader understand how each of these 
variables contributes individually and corporately to the 
formation of cutter roof failure, this section is organized 
on the basis of increasing complexity. Initially, the rock 
mass is modeled as a homogenous, isotropic, linearly 
elastic medium, with only a single opening. This is not an 
accurate representation of the actual mining environ- 
ment, and it is not designed to give any specific indication 
of the characteristics of failure, but it does provide a 
simplified means of demonstrating how stress concentra- 
tions develop along the periphery of mine openings. Once 
the concept of stress concentrations around mine openings 



has been introduced, the two most important variables in 
cutter roof failure formation are discussed in detail, the in 
situ stress environment and rock mass characteristics. 



OPENINGS IN A ROCK MASS 

Once an opening is created in a rock mass, stress 
concentrations form around the periphery of the opening, 
with locations and magnitudes that are a function of the 
shape of the opening, the stress environment (in situ 
stress), and the characteristics of the rock mass (including 
the mechanical properties of the rock). If the in situ stress 
is a function of gravitational stresses alone (excluding 
shear effects and idealizing the medium as a 
homogeneous, isotropic, elastic field), the stress magni- 
tudes can be roughly estimated if the surface topography 
above the area is relatively flat. In this case, the vertical 
stress is a result of the weight of the overburden and is 
calculated as shown in equation 1. 

a, = dD, (1) 

where u,, = vertical stress, lb/ft', 

d = average density of overburden, lb/ft\ 
and D = vertical depth, ft. 

The horizontal stress is also simplified if plain strain 
conditions area enforced, thereby becoming a function of 



the vertical stress and Poisson ratio for the given rock 
type, which reduces to equation 2, after Phillips {41). 



(2) 



O'h - (Tv > 

1-v 

where ah = horizontal stress, Ih/ft', 

a^ = vertical stress, Ib/ft^ 
and V = Poisson ratio for the rock, unitless. 

For this discussion of the influence of opening 
dimensions on the formation of stress concentrations 
around the periphery of mine openings, the in situ stress 
state results from gravitational effects. Isotropic material 
and the effects of linearly elastic properties can be 
assumed, and the deformation of the rock may be analyzed 
as a plain strain system. Since the vast majority of coal 
mine entries in the United States are rectangular in cross 
section, the basic rectangular shape is the only shape 
considered in this discussion. It is shown later in the 
section "Mine Design Changes," that changes from 
rectangular to other shapes may prove to be an effective 
control measure. 

Variations within the fundamental rectangular shape 
can have an effect on stress concentrations along the 
periphery of an opening for a given stress environment, 
thus influencing failure propagation. For this reason, 
isolated single openings will be considered first. Later, the 
interaction of multiple-entry configurations both in 
single and multiple-seam scenarios will be discussed. 

Single Openings 

Figure 5 illustrates an estimation of the stress 
distribution around the periphery of rectangular openings 
having width-to-height (W/H) ratios ranging in value 
from 1 to 3. The figure was constructed from a 
finite-element analysis using the conditions outlined at 
the beginning of this section. In general, the models can be 
interpreted by taking particular note of the areas that 
have the greatest density of contour lines. These areas of 
high stress concentrations are the most probable areas for 
failure. The stress distribution is illustrated by two 
different contour lines: (Da solid line representing equal 
values of the ratio of the maximum secondary principal 
stress, at the points through which the contour passes, to 
the maximum stress applied to the model and (2) a dashed 
line representing the ratio of the minimum secondary 
principal stress to the maximum stress applied to the 
model. 

It will be shown that the location and orientation of 
the failure plane is actually a function of the shear stress 
that develops as a result of the difference between the 
principal stresses. The numbers in figure 5 at the entry 
opening corners, and midspan of roof and floor are 
numerical approximations of the ratio of maximum 
secondary principal stress for that immediate area of the 
model to the maximum applied stress. The values 
calculated for these areas are not to be directly applied to 
the in-mine environment but are provided only to give an 
indication of the increasing magnitude of the stress 
concentrations as the shape of the opening changes. 

Figure 5 demonstrates that for increasing W/H ratios, 
the stress increases in the corners of the entry, while the 
midspan is essentially under no stress. The opening most 
representative of presently operating underground coal 
mines is shown in figure 5C with a W/H ratio of 3 




-(Th 



A. -p- = l 



— <r\y 




"<rh 



Figure 5. — Stress concentration diagrams of rectangular 
openings with width-to-height ratios of 1 (A), 2 (B), and 3 (C) 
under gravitational loading conditions. (Values are ratios, 
representing the major principal stress, for the critical point, 
divided by the maximum stress applied to the model.) 



(although many mines have higher W/H ratios). However, 
it is important to note that the stress concentrations 
around an actual coal mine entry will be influenced by the 
rock properties and stress environment. For the opening 
modeled here, the central roof portion of the entry 
approaches a tension value of stress, and both the central 
rib area and entry corners are subjected to compressive 
stress in excess of the applied vertical stress. Several 
researchers have demonstrated similar characteristics of 
openings under specific load conditions, using other 
techniques (20, 39). 

Multiple Openings 

Since the excavation of a single opening redefines the 
state of stress in a rock mass, it follows that the ex- 
cavation of more than one opening further redistributes 
stresses. Such a redistribution of stresses occurs in (1) the 
mining of multiple adjacent entries in a single seam and 
(2) multiple-seam mining. With the first case, many mines 
that drive several parallel entries experience cutter roof 
failure in entries adjacent to the solid coal. The cause of 
this failure has often been attributed to so-called 
abutment pressures {36). Figure 6 is a finite-element 
analysis of six openings with a W/H ratio of 3, using the 
conditions previously described. The analysis demon- 
strates that the stresses in the upper corners of an entry 
are greatest when that entry is in the center of the panel. 
Theories on the use of pillars for support of roof calculate 
similar results but the in situ stress can vary from the 
ideal case of gravitational loading alone, which can alter 
the location of stress concentrations around openings. In 
most cases, abnormally high in situ stress is thought to be 
controlling the occurrence of failure in these outer entries. 
Under the second scenario, many mines in a 
multiple-seam configuration have experienced cutter roof 
failure. The most common occurrence has been in the 
overlying workings when entries being driven over the 
solid suddenly cross over entries in the lower seam or a 
gob area. Several researchers (16-18, 40) have concluded 
that superimposing of mine workings (i.e., pillar directly 
above pillar, entry directly above entry, crosscut directly 
above crosscut) decreases the stress concentrations around 
the openings and consequently the risk of failure. 
Although this recommendaton has improved mining 
conditions, it too has limitations. In practice it was found 
that even when workings were superimposed, for inter- 
burden of less than 110 ft (in the Appalachian region) (18), 
stress concentrations developed causing roof instability. 

The analyses of stress distributions around the 
periphery of mine openings generally indicate that with 
the enlargement of the width dimension of an opening the 
magnitude of the stress concentrations increases. Addi- 
tionally, with the introduction of adjacent openings the 
magnitude of the stress concentrations increases even 
more. 

STRESS ENVIRONMENT 

Prior to the excavation of an opening in a rock mass, 
states of stress exist in the rock, which are functions of 
gravitational and tectonic forces, thermal stresses, gas 
pressures, and material and rheologic properties of the 
strata. In the United States, in situ thermal stresses have 
negligible influence on the stresses experienced in coal 
mining, and modeling has shown that gas pressures. 




-6 15 /3.44 \ -5.54 ; 3.08 , -6.36 -5.54 344_;-6.l5 
r^^ -6.56^ 



Figure 6. — Stress concentration diagram of six adjacent 
openings under gravitational loading conditions. (Values are 
ratios, representing the major principal stress, for the critical 
point, divided by the maximum stress applied to the model.) 

likewise, have little effect on the formation of cutter 
failure (44). However, forces of tectonic origin and forces 
that are a function of differential loading (as in the stress 
state beneath stream valleys) have significant influence 
on the stability of underground openings. In either case, 
the stress environment surrounding the underground 
excavation is such that the magnitude of the horizontal 
stress is some value greater than what would be 
calculated on the assumption of uniform gravitational 
loading alone. 

A simple analysis of the influence of this increased 
value of horizontal stress is illustrated in figure 7, which 
shows the results of a finite-element model of an opening 
with a W/H ratio of 3, in a two-dimensional, linearly 
elastic, isotropic, homogeneous substance, subjected to an 
increasing ratio of horizontal to vertical stress (cth/o'v)- It 
should be recognized immediately that this is the worst 
case scenario from figure 5; i.e., under gravitational 
loading alone, the opening with a W/H ratio of 3 had the 
least desired stress concentrations of the three openings 
presented in figure 5 (although the model still does not 
directly represent the actual in-mine conditions, owing to 
the absence of realistic rock properties). A familiar trend 
is also seen in figure 7, in that as the horizontal-to- 
vertical-stress ratio increases (from 7A to 7C) the 
magnitude of the major principal stress concentration 
along the upper corners of the entry likewise increases. 

Aggson (4) conducted an analysis of the influence of 
the ratio of horizontal to vertical stress, using actual rock 
properties and calculating the maximum shear stress in 
various models to determine the characteristics of failure. 
He calculated the maximum shear stress because cutter 
roof failure most likely initiates when this value exceeds 
the shear strength of the immediate roof strata. Figure 8 
qualitatively demonstrates his results, showing the effect 
of different vertical-to-horizontal-stress ratios on the 
angle the failure plane makes with the vertical. The 
orientation of the predicted shear fracture rotates out over 
the opening as the horizontal stress component increases. 
Kripakov (27) conducted similar analyses, modeling the 
mine entries of the Kitt Mine of northern West Virginia 
using in situ stress measurements and actual rock 
properties. Figure 9 illustrates the general conditions of 
Kripakov's model and the manner in which the different 
stress values are resolved at the corner of the entry. The 
elemental components of the shear stress are resolved 
from one-half the difference between the two principal 
stresses at any point in the roof. Table 1 lists the various 
values corresponding to figure 9 for different entry widths 



; — o-h 




M M I I M n I M I M 



— o-h 



Figure 7. — Stress concentration diagrams of rectangular 
openings with width-to-height ratio of 3 and loading conditions 
of horlzontal-to-vertical-stress ratios of 1 {A), 2 (B) and 3 (C). 
(Values are ratios, representing the major principal stress, for 
the critical point, divided by the maximum stress applied to the 
model.) 

and other changes to the basic entry shape, which are 
discussed in the section "Mine Design Changes." 

In each of the models developed for this discussion of 
stress concentrations around mine openings, and in the 
models used by Aggson and Kripakov, the rock mass is a 
continuum without separation along bedding planes. 
However, although the in situ stress plays a major role in 
the propagation of cutter roof failure, the characteristics 
of the rock mass also contribute to the end result and are 
necessary input for assessing the formation of cutter roof 



o-h- 




Plane of maximum shear [a\^<ay) 
Poisson effect ^ , 

/-Tension fracture ' 

rCooT 



A , HORIZONTAL STRESS LESS THAN VERTICAL STRESS 



o-h- 




Plone of maximum shear — ~- — 

(ah^cTy' — ^ --■ — 

Hydrostatic — : _ 



B , HORIZONTAL STRESS EQUAL TO VERTICAL STRESS 



C7y 



I I I I I I I it I t M 



Plane of maximum shear 

(cTh > CTy ) 



[Coal 




o-h 



C, HORIZONTAL STRESS GREATER THAN VERTICAL STRESS 

Figure 8. — Qualitative example of influence of horizontal-to- 
vertical-stress ratio on angle of failure propagation. (Courtesy N. 
P. Kripakov) 



Plane of max 
shear stress 




Figure 9. — Analysis of elemental components of angle of 
failure as related to stress environment and rock properties. 
[Adapted from Kripakov (27)] 



failure. Rock properties are discussed later, in the section 
"Rock Mass Characteristics." 



Regional Stresses 

Basically, there are two stress cases that have a direct 
influence on the formation of cutter roof failure in coal 
mines: tectonic stress and differential gravitational stress, 
both of which can often be identified by recognizing 
patterns of failure from a minewide perspective. Control 
measures and evasive measured are different for each 
case. Stresses that are a result of tectonic forces often 
display characteristics that suggegt regional influence and 
are discussed as regional stresses. Stresses that are a 



Table 1. — Maximum shear stress values and analysis of failure 



Condition 



Stress (cr), psi 



IWaximum 



Minimum 



Angle from horizontal 

to maximum principal 

stress (28), deg 



Maximum 

shear. 

psi 


Angle from vertical 
to plane of maximum 
shear stress (if), deg 


1,120 
1,099 
1,022 




19.52 
19.72 
21.45 


919 
744 
693 




41.57 
44.47 
44.38 


1,109 

1,055 

975 




20.54 
24.41 
28,42 



As mined: 

17-ft entry width -930 

16-ft entry width -924 

Pillar softening, 3-tt depth at roof line . . -814 

Slot at roof line: 

3 by 12 in 68 

3 by 24 in 11 

3 by 36 in 42 

Slot at pillar midheight: 

3 by 12 in -868 

3 by 24 in -638 

3 by 36 in ^19 

Source: Adapted from Kripakov (27). 



-3,170 
-3,121 
-2,857 

-1 ,770 
-1 ,476 
-1 ,343 

-3,085 
-2,748 
-2,368 



64.52 
64.72 
66.45 

86.57 
89.47 
89.38 

65.54 
69.41 
73.42 



result of differential gravitational loading are discussed 
under the heading "Stress Concentrations Beneath 
Stream Valleys." 

Stresses of tremendous magnitude have been known 
to exist in the Earth's crust ever since the earliest 
geologists recognized that mountains are the result of 
massive regional uplift. The fact that the horizontal 
component of these stresses is rather uniform with respect 
to orientation, over vast areas of continents, can be easily 
demonstrated by the curvilinear trend of the Appalachian 
Mountains or most other mountain chains. In coal mines 
of the United States, the influence of these regionally high 
in situ horizontal stresses has been recognized since at 
least the 1930's (42) and is currently the focus of much 
research. 

Probably the most conspicuous and most often cited 
sjrmptom of regionally high horizontal stresses in coal 
mines is that cutter roof failure occurs most frequently in 
one particular orientation (i.e., in either the main 
headings or the crosscuts but not both). References to this 
phenomenon in the United States often make note of the 
fact that this type of unidirectional roof failure occurs 
mainly in northerly oriented headings (7-8, 25, 29). In 
many cases, it has been found that this northerly direction 
is subperpendicular to the major principal horizontal in 
situ stress. Figure 10 demonstrates the regional presence 
of excessively high horizontal in situ stress across the 
United States as compiled by Aggson (2). Table 2 gives the 
actual values of the stress for the various locations. Of 
particular interest is the fact that most of the readings 
revealed a horizontal biaxial stress field with a maximum 
principal stress greater in magnitude than would be 
calculated in the usual manner (equation 2). [Engelder 
(13) has compiled more recent in situ stress data for the 
northeastern United States.] 

Figure 11 is a map of a portion of a mine in western 
Pennsylvania showing the locations of roof falls. The fact 
that the majority of roof falls occur along the same 
orientation suggests the influence of regionally high 
horizontal stresses where the difference in magnitude 
between the minimum and maximum secondary principal 
stresses is relatively large. Recognition of patterns of roof 
failure such as this can aid in determining whether the 
failure is a local or regional phenomenon and can 
eventually aid in selecting a control measure. 

In other instances where the in situ horizontal stress 
state is greater than the expected value from gravitation- 
al loading, there is little difference between the magni- 
tudes of the principal horizontal stresses. An example of 
this was reported on separately by Aggson (3) and 
lannacchione (23). Aggson conducted in situ horizontal 




Sell*, pal 



Figure 10. — Horizontal in situ compressive stress measure- 
ments in the United States. Stress fields are drawn to pressure 
scale (in legend), not to mileage scale. [Adapted from Aggson (2)] 



stress measurements by overcoring and found the horizon- 
tal stress to be at least three times the vertical stress at 
the Kitt Mine in northern West Virginia. However, only a 
22-pct difference was found between the minor and major 
principal horizontal in situ stresses. lannacchione found 
little difference in the frequency of failure in crosscuts 
versus entries, through in-mine mapping that reconfirmed 
that the orientation of the entries was not influencing the 
formation of failure. Kripakov (27) also reported on cutter 
roof failure at the Kitt Mine. His research indicated that 
modification to the pillars, such as cutting slots in the 
pillar near the roof, would be necessary to effectively 
control the cutter problem (discussed further in the 
section "Mine Design Changes"). In short, it is very 
difficult to control cutter roof failure when the cause is 
regionally high horizontal stresses and especially when 
there is little difference in magnitude between the 
maximum and minimum horizontal stresses. 

Stress Concentration Beneath Stream Valleys 

Many mines that normally have relatively good 
ground conditions occasionally encounter severe condi- 
tions in the form of cutter roof failure. These localized 
occurrences can be the result of stress concentrations 
beneath stream valleys, mining-induced stresses, or rock 
type. In addition, localized high-stress conditions exist in 



10 



Table 2. — Horizontal in situ stress measurements, surface and underground sites 



Site' 



Location 



Direction 
of P 



Magnitude 
of P, psi 



(vlagnitude 


Average deptli 


of Q. psi 


of measurement^ft 


941 


18.1 


285 


1.8 


1,191 


33 


1.385 


8.6 


335 


4.8 


1,113 


61.9 


516 


1.2 


791 


151.2 


1,397 


4.7 


1,519 


4.9 


777 


4 


519 


4.5 


1,491 


4.7 


171 


10 


551 


925 


2,500 


2,300 


2,937 


3,200 


1,595 


1,000 


1,053 


6,200 


4,966 


5,300 


2,283 


3,127 


2,898 


1,060 


404 


1,600 


705 


850 


345 


1,250 


160 


1,570 


1,466 


700 



Surface: 

1. .. 

2. .. 

3. .. 

4. .. 

5. .. 



9. 
10. 
11. 
12. 
13. 
14. 



Lithonia, GA 

Dougiasville, GA 

Mt. Airy, NC 

Rapidan, VA 

St. Peters, PA 

West Cfielnesford, IVIA 

Proctor, VT 

Barre, VT 

Graniteville, IVIO 

St. Cloud, (WIN 

Carthage, MO 

Troy, OK 

Marble Falls, TX 

Green River, WY 



Underground: 

15 Immel Mine, Knoxville, TN 

16 Limestone Mine, Barberton, OH 

17 Mather Mine, Ishpeming, Ml 

18 Fletcher Mine, Bunker, MO 

19 Homestake Mine, Lead, SD 

20 Crescent Mine, Wallace, ID 

21 Henderson Mine, Empire, CO 

22 Sunnyside. Mine, Sunnyside, UT 

23 Allied Chemical Mine, Green River, WY 

24 Big Island Mine, Green River, WY 

25 Rainier Mesa, NV, test site 

26 Lakeshore Mine, Casa Grande, AR 

27 Beckiey No. 1 Mine, Bolt, WV 



49° E 
64» W 
87° E 

6° E 
14° E 
56° E 

4° W 
14° E 
77° E 
50° E 

2° E 
84° W 
33° W 
42° E 



N 58° E 

N 77° E 

N 82° W 

N 17° W 

N 38 

N 27 

N 15' 

N 31' 

N 23' 

N 38' 

N 46° W 

N 69° E 

N 69° E 



1,639 

512 

2,464 

1,678 

820 

2,133 

1,328 

1,734 

3,190 

2,205 

1,066 

1,075 

2,219 

415 



3,007 
4,000 
3,822 
3,682 
2,778 
6,258 
3,398 
3,718 
1,781 
1,054 
972 
502 
2,973 



'Numbers correspond to those on figure 10. 

NOTE. — P and O are the maximum and minimum secondary principal stresses. 
Source: Adapted from Aggson (2). 




LEGEND 

' Roof falls 





L 



800 



Scale, ft 



Figure 1 1 . — Partial mine map from western Pennsylvania, showing preferential orientation of roof falls. 



11 




Streams 



Figure 12.— Mine map from mine in southern. West Virginia, siiowing correlation of roof fall locations with centers of overlying 
stream valleys. 



areas of close proximity to anomalous geologic structures 
such as paleochannels, rolls, and clastic dikes. For the 
purpose of emphasis, the only topic discussed in this 
section is the effect of surface topography on underground 
opening stability. The influence of anomalous geologic 
structures is discussed under the heading "Minor Geologic 
Structures." 



Figure 12 is a map of a mine in southern West 
Virginia showing roof falls and overburden. The pattern of 
roof falls beneath the stream valleys is typical of 
valley-stress-induced failure. All other factors in the mine 
remained relatively constant, such as rock type and 
opening dimensions; thus, differential gravitational stress- 
es were deduced as the cause of failure. For many other 



12 



mines, similar correlations can be found between stream 
valleys and roof failure, although failure may occur in 
wider or narrower zones throughout the mine. In still 
other cases, mining beneath a valley may create no 
adverse mining conditions. From the limited amount of 
research conducted on this phenomenon, the following 
variables are thought to influence the occurrence of cutter 
roof failure associated with stream valleys: rock type, 
opening dimensions, percent extraction, depth beneath 
the valley, relief of surface topography, gradient of valley 
walls, availability of flowing water, and magnitudes and 
orientation of the principal in situ stresses that influence 
differential gravitational loading. It is also important to 
note that, depending on the variables just listed, failure 
may occur in ways other than cutter roof 

It has long been known that mining under low cover 
beneath stream valleys creates a potential for bad ground 
conditions. Little in-mine research has been conducted to 
analyze the cause, as a guide for developing control 
measures, although it has been estimated that as much as 
90 pet of all roof falls in the northern Appalachian Basin 
occur beneath valleys (36-37). Ferguson (14-15) described 
unstable ground conditions associated with valley stresses 
in almost all surface engineering work within valleys in 
the Appalachian Plateau region. Laboratory investiga- 
tions have revealed that a stream valley can be modeled 
as a V-notch in a horizontal, thick plate subjected to 
gravitational loading. Lang (28) demonstrated the effect 
of a stream valley on the in situ stress environment using 
a V-notch in a photoelastic gelatin model, as did 
Worotnicki (50), who also used an electric analog model. 
These models showed that immediately beneath a 
V-notch, and for a substantial depth (as much as 600 ft 
beneath it), the horizontal stress is actually greater than 
the vertical stress (compared with the case of uniform 
gravitational loading beneath relatively flat topography). 
If other stresses are applied, the stress concentrations are 
modified by the notch. 

Wang (48) used a two-dimensional, finite-element 
model to compare the stress concentrations around an 
opening beneath a stream valley with the stress concen- 
trations around an opening beneath an adjacent hill. For 
this analysis, the only applied stress was gravitational 
loading, and the mine openings had a W/H ratio of 2. The 
coalbed of the model was attributed a different modulus of 
elasticity from the rest of the model to more accurately 
represent the in situ environment. Figure 13 is a 
comparison of the results for each of three sets of openings 
as the height of the hill increased from 240 to 355 to 615 ft. 
(The depth of the opening beneath the valley remained 
constant at 115 ft.) The most important feature of this 
graph is that the opening beneath the valley has higher 
compressive stresses in the corners and lower tensile 
stresses in the midspan of the roof than does the opening 
beneath the hill. 

Moebs [as cited by Enever (12)] conducted a study on 
the frequency of roof falls as related to their lateral 
distance from the center of valleys, compiling data from 
several mines in the northern Appalachian Basin. Based 
on his data, Moebs developed an empirical method of 
determining the likelihood of roof instability associated 
with a stream valley as related to the slope of the valley 
walls and the depth of the mine below the valley. 
Equation 3 represents his empirical method. 



550 



Pillar 
compression 




Pillar 
compression 



KEY 
Opening under hill 

Opening under valley 

Roof corner 



-Midspan 
tension 



Midspan 
tension 



Roof corner 
ompression 



300 



500 



700 



900 



HEIGHT OF HILL, ft 



Figure 13. — Values of stress for critical points of an opening 
beneath a valley versus an opening beneath a hill, for an 
increasing height of the hill. [Adapted from Wang (48)] 



where 



and 



F = an empirical index, unitless, 

A = the mean slope angle of one of the valley 

walls from the horizontal, deg, 
A = the mean slope angle of the other valley 

wall from the horizontal, deg, 
D = the depth of the coalbed at the point of 

interest, beneath the valley wall, ft. 



F (fall factor) = ( A + A' ) 
D 



(3) 



It can be seen from the equation that as the steepness of 
the valley walls increases, the values of F correspondingly 
increase, representing an increasing risk of roof falls. 
However, the F value decreases for greater depths. 
For the small set of mines analyzed by Moebs, a particular 
value for F was obtained and a greater F value in any area 
usually indicated imminent failure. Moebs did not 
specifically identify this "critical" value since it has not 
been shown to be universal in application. Another factor 
discussed by Moebs in a separate publication (37) is the 
fact that the highest frequency of falls at each mine 
occurred beneath streams oriented in a northerly direc- 
tion. As mentioned earlier, for the northern Appalachian 
Basin this northerly direction is subperpendicular to the 
major principal horizontal in situ stress. Thus, it is 
assumed that stream valleys with a northerly orientation 
increase the magnitude of the horizontal in situ stress. 

This phenomenon of the influence of the stream 
valleys on the already high horizontal stress field was also 
seen in Australia by Enever (11), where in situ stress 
measurements revealed not only an increase in magni- 



'13 



tude but also a reorientation of the principal stresses. The 
research showed that a ratio of depth of cover to the 
maximum surface relief for a particular valley (D/R, 
where D is the depth and R is the maximum surface relief 
with compatible units) was the most reliable empirical 
relationship for determining the likelihood of valley- 
stress-induced failure of underground workings. The 
value of D/R is calculated at any point within the valley to 
determine the likelihood of failure beneath that point. 
Enever concluded by suggesting that D/R ratios of less 
than 0.5 indicate a strong possibility of encountering 
adverse roof conditions, and that this ratio would need to 
be adjusted in the presence of regionally high horizontal 
stresses. 

Both Moebs and Enever admit that their empirical 
methods are unable to treat the in situ stress state as a 
separate variable. Additionally, a second missing variable 
may be one that would take into account drastic changes 
in the cross-sectional profile of the valley as taken 
perpendicular to the trend of the valley. It is important to 
note that these empirical relations are based solely on 
observations of physical conditions and do not include the 
influence of the failure mechanisms. Further, neither of 
the equations has been tested against a statistically 
significant population of failure occurrences. For this 
reason, these methods are presented only as a means of 
demonstrating the problem to the reader. For predicting 
failure, these relations would most likely need to be 
adjusted from mine to mine; e.g., they should also include 
any pertinent information concerning failure in the area 
of question, such as variables of available water, rock 
type, and geologic anomalies. 

ROCK MASS CHARACTERISTICS 

Rock mass characterization is a vast subject area 
concerned with the comprehensive description of rock 
masses. The characterization of rock mass has been 
tackled by many researchers through the use of classifica- 
tion systems [of which Bieniawski (6, pp. 97-132) has 
listed the most popular presently being used]. Each of the 
various systems attempts to classify a rock mass either 
qualitatively or quantitatively into groups exhibiting 
similar behavior. A number of parameters are utilized in 
the various classification schemes (e.g., compressive 
strength of the intact rock and joint spacing), with each 
variable being weighted in terms of its overall importance 
with respect to support. The descriptions and value of 
these systems in mine design are not discussed in this 
report; however, the two most common threads in these 
systems are significant factors in the propagation of cutter 
roof failure: rock properties (including elasticity) and 
minor geologic structures (such as joints, clastic dikes, and 
facies changes). 

In the previous discussions of the effect of opening 
dimensions and stress environment on cutter roof prop- 
agation, all other variables were held constant at some 
value of either a commonly encountered condition or 
convenience with respect to modeling. In each case, it was 
useful either to omit the variable of rock mass characteris- 
tics by using a constant of purely elastic uniform 
homogeneity, or to only partially incorporate it by using 
the elastic properties of the rock. For coal measure rocks, 
this is obviously not an accurate description. In this 
discussion of rock mass characterization, the in situ stress 
state is held constant at a value of gravitational loading, 
and the opening dimensions have a W/H ratio of 3. 



OO 

cn >_ 

COUJ 



25 


— Harder coal 


Softer coal—* 


1 1 


1 I 


20 

- 1 c^ 


1 1 


1 1 



I/I 



1/2 1/4 1/6 1/8 1/10 



RATIO OF MODULUS OF ELASTICITY OF COALBED |Ec) 
TO MODULUS OF ELASTICITY OF ROOF ROCK (Epl 

Figure 14. — Stress concentration in roof-rib corner versus 
elasticity of coalbed. (Stress concentration values are ratios, 
representing the major principal stress, for the critical point, 
divided by the maximum stress applied to the model.) [Adapted 
from Wang (47)] 



However, as the influence of separate rock mass charac- 
teristics is analyzed, it will become clear that the geologic 
environment places limitations on opening dimensions 
and at times creates local stress anomalies. 

Rock Strength And Stiffness 

Wang {48) used finite-element analysis to investigate 
the effects of rock stiffness (or elasticity) on the stress 
distribution around single mine openings. For the first 
analysis, he looked at a single opening in a coal seam 
bounded by a uniform shale above and below, from which 
he concluded the following: 

The stresses in the mine roof are highly 
dependent on the relative values of the elastic 
moduli of the roof materials closest to the surface 
of the opening. Where the roof is a single 
material, the compressive and shear stresses at 
the roof-rib intersection tend to decrease and the 
tensile stress at midspan of the roof tends to 
increase as the roof material becomes stiffer 
elastically in respect to the coal seam .... 

The converse of this statement also holds true: As the coal 
seam becomes stiffer elastically in respect to the roof 
material, the compressive and shear stresses at the 
roof-rib intersection tend to increase and the tensile stress 
at midspan of the roof tends to decrease. Figure 14 
illustrates how the magnitude of the stress concentration 
changes as coal elasticity changes. 

Wang went on to analyze single mine openings in 
multilayered material, which had primarily been ana- 
lyzed with beam equations in the past. Obert (39) used the 
beam method of analysis to estimate limits of stable roof 
spans across single and multiple entries in multilayered 
material. The results of Wang's work, however, illustrate 
with simplicity the qualitative interpretation of opening 
stability for multilayered roof (48): 

... for a multicomponent roof, both the shear 
and compressive stresses at the roof-rib intersec- 
tion and the midspan tensile stress increase in 
value when the elastically stiffer roof material is 
closest to the surface of the opening. 



14 



600 



500 



400 



300 



200 



100 



1 

KEY 
• Tensile strength 
A Compressive strength 




J I I 1 L 



1/6 3/12 1/3 5/12 1/7 2/3 
TI/(TI + T2) 



5/6 



900 



800 



700 



600 



500 



400 



Bock to in SITU 
sheor stress vtilut 



So: 

O UJ 



Of?, 



Z3 W 



Figure 15. — Stress values in roof-rib corner and midspan of 
roof versus ratio of thickness of the two immediate roof 
members. T^ Is the roof rock layer closest to the opening, and Tj 
is the layer immediately above T,. [Adapted from Wang (48)] 

Wang further defined the stability of openings in 
multilayered material by analyzing the thickness of the 
various roof members, the results of which are illustrated 
in figure 15. The figure demonstrates that the threat of 
cutter failure is greatest for thick, weak layers of rock that 
overlie a strong immediate roof 

When only rock strength is considered as the 
controlling factor in the propagation of cutter roof failure, 
the same worst case scenario can be drawn from both the 
finite-element and beam methods of analysis. In 1950, 
Thomas {45) reported the same findings as Wang's based 
on underground observations of the cutter roof failure 
process and described the worst case scenario as follows: 

. . . the conditions necessary to produce a 
"cutter" are: (DA relatively strong immediate 
roof that may be thinly laminated, but the 
cementation between the laminations must not 
break down easily, and (2) a series of weaker 
strata that tend to sag and slowly load the 
immediate roof below it. 
In addition, as the thickness of the strong immediate roof 
member decreases, the effect of loading upon this member 
by the overlying weaker member increases. 

The work of Aggson (4) and Kripakov {27) is discussed 
in the section "Stress Environment" with respect to the 
orientation of the cutter fracture plane in the roof as a 
function of the in situ stress. However, the actual 
calculations for exact determination of the location and 
orientation of the fracture plane for a particular in-mine 
condition are extremely complex; the required data for 
calculating these values are the elastic properties of the 
rock, the stress concentrations around the periphery of the 
opening (under the given in situ stress environment), and 
the manner in which these stresses are redistributed as 
the fracture propagates. This information is useful in 
calculating the failed roof rock load on artificial support. 
As an example, when trusses are used to support failed 
roof it is beneficial to know how much dead weight is to be 
suspended in order to use a sufficiently large gauge of 
steel. 

Minor Geologic Structures 

A minor geologic structure inherent to coal measure 




Figure 16. — Qualitative interpretation of cutter failure prop- 
agation in thinly bedded, single roof rock type. 

rocks, introduced in the discussion on rock strength and 
stifftiess, is the naturally occurring bedding planes of 
sedimentary rocks, which commonly separate rocks of 
differing material properties. These discontinuities divide 
the roof rock into separate beams, which allow for shear 
displacement as the roof sags into the mine opening 
(provided the rock does not sag beyond its elastic limit). 
The fewer the number of bedding planes, the greater the 
horizontal shear stiffness of the roof material. From 
available beam equations, it has been shown that as the 
shear stiffness of the roof increases, the predicted point of 
failure moves closer to the ribline {24). Conversely, as the 
number of bedding planes in the roof increases, the shear 
stiffness decreases and the predicted point of failure 
moves toward the center of the entry. Thus, according to 
beam equations, if horizontal shear stiffness were the only 
controlling factor in the occurrence of cutter roof failure 
for a specific site, the worst case scenario would be a 
massive roof rock unit devoid of bedding planes. However, 
roof rock, of only one rock t5T)e, with many weak bedding 
planes has also been observed to fail in the cutter manner 
{29-30). 

Figure 16 qualitatively demonstrates the possible 
mechanisms behind the failure of thinly laminated rock 
that does separate between bedding planes as a result of 
differential deformation along individual beds. When the 
mine opening is initially excavated, the maximum shear 
stress is in the roof rock layer closest to the opening 
(labeled 1 in figure 16); as a result, this unit undergoes 
slightly more deformation than does the overlying layer, 
causing a gap to form between the layers. The shear stress 
causes this lowest layer to fail, and a redistribution of 
stress occurs, creating an unstable environment for the 
next layer up. This process continues until an equilibrium 
is reached, usually in the form of massive roof failure. 
This type of roof failure has been frequently observed 
underground, and Aggson (2) has identified and described 
a similar failure mechanism in floor heave. 

Another minor geologic structure influence would 
appear to be coal cleat. Thomas {45) indicated that cutter 
roof failure occurred most frequently in entries oriented 
parallel with the face cleat, implying some inherent 
relation between cleat and the formation of cutter failure. 
However, present trends in cutter roof failure do not show 
preference to entries oriented parallel with the face cleat; 
in fact, cutter failure generally occurs more frequently in 
headings parallel with the butt cleat of the coal. In either 
case, coal cleat has not been established as an instigator of 
cutter roof failure. The only apparent influence it may 



15 




Figure 17. — Clastic dii<e in coal pillar. Dilie is outlined with white chalk. 



have results when coal is left as an immediate roof 
member; in this case, cleat has an effect on the elasticity of 
the immediate roof Discordant joints (not bedding planes) 
in roof rock other than coal also appear to have little or no 
effect on the formation of cutter roof failure. 

Clastic dikes (or clay veins) have recently been cited 
as significant contributors to the propagation of cutter roof 
failure (19, 23). Figure 17 is an example of a clastic dike in 



a coal pillar; the shape of these minor geologic structures 
may vary significantly over the length of the dike, which 
may or may not cut entirely through the coal from the roof 
to the floor. The width may range from as thin as a 
film-like trace to as thick as SVi ft or larger, and the 
material that infills the dike ranges from claystone to clay 
matrix with inclusions of coal, shale, sandstone, etc. The 
dikes are frequently associated with a minor fault fracture 



16 




Figure 18. — Clastic dike In midspan of roof rock of crosscut, showing how the roof is divided into two separate cantilever beams. 



in the coal and slickensides in the roof, which have been 
observed to extend as much as 25 ft above the coalbed. 
Chase (9) discusses the various features and theories of 
formation of clastic dikes with recommendations on 
support methods. His work shows that clastic dikes are 
associated with many forms of roof failure in addition to 
the cutter type. Figure 18 is an example of the manner in 
which a clastic dike can affect roof rock stability. In the 
case shown, the clastic dike divides the roof span into two 
separate cantilever beams. When the roof is left unsup- 
ported, the shear stress in the corners of the entry 
increases because of the moment imposed by the weight of 
the unsupported beam. Depending upon the stress 
environment and the rock type, the failure that results 
may take on the form of cutter roof failure. 

At the Kitt Mine, lannacchione (23) conducted 
in-mine mapping of minor geologic structures and 
deformation due to pressure release. He found that clastic 
dikes frequently formed the boundaries of cutter roof falls 
and otherwise destabilized roof conditions, allowing cutter 
failure to form. In a study by Hill (19), geologic mapping 
was conducted of the Greenwich North and South Mines 
in Indiana County, PA, and cutter failure was observed to 
form at the point where clastic dikes intersected the 
ribline. And, as in lannacchione's work, clastic dikes were 
also found to form the boundaries of many cutter roof falls. 
Figure 19 is a portion of the mapping conducted at the 
Greenwich North Mine, illustrating the high frequency of 
occurrence of clastic dikes and their association with 
cutter roof failure at this mine. 

Figure 20 is a map of the Eastern United States, 
showing each of the major coal basins and the distribution 
of clastic dikes by their occurrence in mines. A survey of 



mines currently being conducted throughout these basins 
is revealing a similar distribution pattern of mines having 
a high frequency of cutter roof failure. The conclusion 
drawn from this correlation is not that clastic dikes are 
the cause of cutter roof failure but rather that they act as a 
discontinuity from which failure can initiate. 

Other minor geologic structures that affect the 
formation of cutter roof failure are paleochannels and rolls, 
somewhat larger in size than clastic dikes. These 
sedimentary and compactional features can be areas of 
abrupt change in rock type and frequently influence stress 
concentrations around nearby openings. Several geolo- 
gists have described these features in detail, discussing 
their effects on opening stability and suggesting methods 
of support (21-22, 26, 35-36). Severe cutter roof failure 
conditions have also been observed in crosscuts driven 
subparallel to the strike of a roll in a mine in southern 
West Virginia. Figure 21 is a map of the section of the 
mine experiencing problems, and figure 22 illustrates the 
attitude of the roll. In figure 21, note that failure was 
probable where a crosscut was driven directly beneath and 
parallel with the roll. 

Wang (48) conducted finite-element analysis of rolls 
overlying mine openings and found an increase in stress 
concentrations at the corners of the entry. Additionally, 
for entries directly beneath rolls, stress concentrations at 
the roof midspan and corners were calculated for different 
values of the pitch of the roll (or the angle the bedding 
makes with the horizontal). Figure 23 is a graph of Wang's 
results. 

Geologic mapping of mines experiencing cutter roof 
failure is imperative if the causes of failure are to be 
discovered and controlled, but it has been conspicuous by 
its absence in past investigations of cutter roof failure. 



17 




LEGEND 
Clastic dike ^:= Cutler 
Fall area ^s Floor heave 

Kettle bottom 



Scale, 



Figure 19. — Geologic and roof failure map of Main A of Greenwich North IVIine. 



^ LEGEND 

J ^\J etc. Indicatestheoccurrenceof clay veins 
and designates which coalbed is 

^ being extracted 

[Zj,[Z],[Z],6'c. Indicates that clay veins were not 
observed in extracted coolbed 






« Desig- 
nation 

I 
2. 

3. 
4 
5. 
6 
7 
8 
9. 
10 
II 
12 



Mine location 
Coal basin 
Coalbed 



Blue Creek 
Campbell Creek 
Chilton 
Eagle 

Eagle No. 2 
Elkhorn, Lower 
Freeport, Lower 
Freeport, Upper 
Harlan 
Hortshorne 
Hortshorne, Lower 
Herrin 



« Desig- 
nation 

13. 

14. 

15. 

16. 

17 

18. 

19. 
20. 
21 . 
22 
23. 
24 



Coalbed 





High Splint 
Kittanning, Lower 
Kittanning, Upper 
Pittsburgh 
Pocahontas No. 3 . 
Pocohontas No. 4 K 
Pond Creek I 

Powellton, Upper 
Redstone 
Sewell 
Sewickley 
Springfield 



Figure 20. — Clastic dike in-mine occurrences in Eastern United States. (Courtesy F. E. Chase) 



18 



g G D y □D^ODfl^ 

g n D wouooacjo 
p D ggDQ^^^3^ 

EAGLE COALBEp 



Mi 



BBgR 



^i§ 



gpaC 

-"■-in 



aaaDQQ 

DDaa 



I ^ 



--""-f 



f^= 



/ 



LEGEND 
.'' Sandstone washout 

ra Roof fall 
- Roll 



200 



Scole, ft 



'J^'-V-'-- 










nDdg 

dS 
a 



□ ddDD 



^, , .DODDDaDD^anDDDDDDD 

f-,nnnnr-' L't- nnnrfctdyy 




fDDD 






□ DD 
ODD 

m 




[3Y]nDi:7Dac7^zi:?ODac7Dan 00000 c/jaaDDaDannDDC 



NORTHEAST MAINS 



Figure 21.— Section of mine in southern West Virginia, showing iocation of coaibed roii and iocai roof falls. 



5=7.6 




Shale roof 
'Coal 

"""■Shale parting 
Coal 






- 






5 


1 




1 





25 




50 




Scale, 


fT 





Figure 22.— Cross section of roll A-A' from figure 21. 



19 



1,600 
1,200 



in 
a. 



(O 800 

UJ 

a: 

\- 

400 - 







Tension 



Jl 



-• - 



Compression - 



KEY 

■Roof midspan 
Roof corner 



0.10 



0.15 



0.20 



0.25 



PITCH OF ROLL 

Figure 23.— Stress values for critical points of an entry 
versus severity of pitch of an overlying roll. [Adapted from Wang 
(48)] 



USING PREDICTION OF FAILURE IN MINE DESIGN 



For particular cases of cutter roof failure, the causes 
can be determined, as demonstrated in the preceding 
section. During the design stages of mining, one may 
include the possibility of encountering cutter roof failure 
by recognizing the extent to which the factors of 
propagation may influence a particular area. On an 
elementary level, stream valleys that overlie the coalbed 
can be avoided, or a rough estimation of their influence 
may be assessed through the use of the empirical formulas 
previously presented. If information on the state of the 
in situ stress is available, entries can be oriented so that 
the in situ stress has the least effect on ground stability. 
Additionally, a comprehensive analysis of the minesite 



can be conducted if meaningful data on rock properties 
and in situ stress can be obtained and then incorporated 
into numerical models. 

Any specific prediction of cutter roof failure, with 
respect to location, is very difficult. However, if obvious 
factors are accounted for in the design process, changes to 
the design may not have to be implemented at a later date. 
The following section on control measures includes 
subsections on mine design changes and indirect control 
measures. These can be integrated into the initial design 
(when conditions warrant), effectively decreasing the risk 
of exposing miners to cutter roof failure. 



SELECTING CONTROL MEASURES 



Before a control method for cutter roof failure is 
selected, the cause or at least the pattern of failure should 
be determined, if possible. For example, if it can be 
determined that failure is occurring only in localized 
areas of the mine, it is highly probable that control 
methods will only need to be applied locally. On the other 
hand, if failure is the result of regionally high, horizontal, 
biaxial stresses, it may be more effective to reorient mine 
headings as opposed to using supplemental support. 

Figure 24 is a decision process diagram demonstrat- 
ing a suggested method of analysis for determining the 
cause of cutter roof failure at a particular mine. The 
decision process is based on observations of underground 
failure, geologic conditions, and surface topography. Once 
the cause has been identified, the rock mass as a whole is 
analyzed to determine the influence of characteristics of 
the rock that may not be as readily apparent. Once a 



control measure is implemented, careful monitoring is 
conducted to assess its performance. If the performance is 
poor, further investigation is necessary to determine the 
mechanisms involved and further modifications should be 
made to the support technique. 



ARTIFICIAL SUPPORT 

Support of cutter roof failure in the past has been 
directed mainly toward the resupport of roof that has 
already begun to fail. This technique has proven to be 
unsatisfactory. The only resupport methods even margi- 
nally successful are (1) extensive cribbing {19, 47) and (2) 
a combination of additional bolting, trussing, and resin 
injection (3, 5, 19); both of these methods are prohibitively 
expensive, and cribbing often results in the loss of a 



20 



Begin 







No 


Is failure 


of the 






cutter roof type? 






|Yes 






Mine wide 


Is failure localized 

or mine wide in 

occurrence? 


Local 












1 


r 








i 








Does failure show 
a preferred 
orientation? 


Yes 




Are areas of failure 
beneath stream 

valleys' 


Yes 


Design mine for 

limited exposure 

to these areas and 

employ artificial 

support. 














No 




i 




Ino 












Follow recommen - 
dationsfor strongly 
biaxial high horizon- 
tal stresses. I.e., entry 
orientation. 




Is failure in areas 

affected by multiple 

seam mining? 


Yes 


Follow recommen- 
dations for multiple 
seam mining support. 


- 






i Continue 






No 












Analyze rock mass 
characteristics to deter- 
mine influence on 
failure. Modify support 
based on findings 




Is failure mainly 
isolated to entries 
adjacent to the solid? 


Yes 


Consider design 

options for reducing 

number of adjacent 

entries. 


— 
















f 






,No 












Implement control 
method. 




Continue 






i 




neg 


f 
dtive 


Analyze results. 





Figure 24. — Decision process diagram for determining the cause of cutler roof faiiure and selecting control measures. 



passageway or at least an increase in airflow resistance. 
Recent studies have concentrated on the prefailure control 
of cutter roof iailure, with limited success, and a large 
portion of the methods developed in these studies remain 
to be field-tested. The following discussion of control 
methods presents the use of artificial support and mine 
design changes as attempts to control the occurrence of 
cutter roof failure rather than to resupport failed roof 

Angle Bolting 

Angle bolting was one of the first artificial support 
methods employed to inhibit the formation of cutter roof 
failure. During the early 1950's, when roof bolting was 
being promoted for the first time in the coal mining 
industry, one of the major selling points for the use of 
angle bolting was its effectiveness in controlling cutter 
failure development (45-46). Figure 25 presents the two 



most commonly employed types of hardware associated 
with angle bolting. Figure 26 shows the plan of 
installation that would be used when angle bolting is a 
part of the normal roof control plan. When a tensioned 
angled roof bolt is placed near the corners of the entry, the 
shear stress in the corners is redistributed, effectivelely 
decreasing the unsupported span of the entry. With a 
reduction in the shear stress, the probability of failure is 
reduced. Problems associated with angle bolting stem 
mainly from the availability of equipment needed for 
drilling inclined bolt holes (an angle of 45° with the 
horizontal is recommended). The use of handheld 
pneumatic drills for installation is awkwar/l and time 
consuming. In addition, if the bolts are not installed 
on-cycle, shortly after mining of the cut, the roof may sag 
significantly, creating dangerously high shear stresses in 
the corners of the entry. Dual-boom tilt-head bolters are 
available that make the on-cycle installation of angle 
bolts a relatively routine operation (5). 



21 





A ROCKER NUT TYPE B, ANGLE IRON TYPE 

Figure 25. — Angle bolting hardware. 





^~ _,„-^ — 


^^^-;^ 


j— 




.:^^_'^_'^ J2= ^ 


- ,-v-- -->«^r-^ 


=^ 


fc=^ Shale — 
xs 




^* 


^ 


^^^S[ 


— ^^^ 4 


r A 

— 4'— 

1 o' 


1^ '^ 

— 4'- +3'- 


1 


v////y///// 


lo 





A CROSS SECTION VIEW 

10 

I I I 

Scale, ft 



----E H Y S & 

4' 

Shale 
roof 

A- — 

B, PLAN VIEW 
Figure 26. — Angle bolt installation. 



Truss Bolting 

Truss bolting is an artificial support technique 
commonly recommended for cases of severe cutter roof 
failure not significantly deterred by the installation of 
angle bolts (5, 36). The two most commonly used designs of 
truss bolts are shown in figure 27, along with an 
indication of the theoretical components of compressive 
force that make this method of support successful. 
Controversy over the comparative effectiveness of the two 
separate designs has arisen from the ability of the truss in 
figure 27B to maintain equal tension in both the 




A ANGLE BOLT TYPE 




B, CONTINUOUS BOLT TYPE 

Figure 27. — Two basic designs of roof bolt trusses. Arrows 
shown in A represent compressive forces which are also true 
forB. 



crossmember and bolts, while the truss of figure 27A 
allows for adjustments in these tensions after installation 
(29-33). An advantage to the truss type of figure 27A is 
that the crossmember does not have to be installed 
immediately and can be used intermittently as conditions 
warrant. The normal plan of installation for both types of 
truss bolts is shown in figure 28A, but some mines have 
gained amendments to their ground control plans allow- 
ing them to use the angle bolt portion of the truss (shown 
in figure 27A) on-cycle as a replacement for the outer two 
bolts. The installation of the crossmember then qualifies 
as supplementary support when hazardous conditions are 
encountered (fig. 28B) (5). This has proven to be 
exceptionally useful in areas where cutter roof initiates at 
clastic dikes. In-mine monitoring of roof behavior near 
clastic dikes at the Greenwich North Mine showed that 
trusses were most successful when installed on-cycle (19). 
Again, the obvious drawback to truss bolting is the need 



22 



Entry 



Entry 



A SUPPLEMENTAL SUPPORT B, ON-CYCLE INSTALLATION 

20 

I I I 



Scale, ft 
Figure 28. — Plan view of truss installation. 

for a bolting machine that can drill inclined bolt holes (the 
recommended angle of installation is 45°). Installation 
on-cycle is also recommended for the reasons discussed 
under the heading of "Angle Bolting." 

Other Supports 

As mentioned earlier, cribing and posts are generally 
used for the resupport of roof after cutter failure has 
formed; however, in specific cases where the failure begins 
at a clastic dike or some other minor geologic structure 
and in situ stresses are not too high, strategic placement 
of cribbing and posts has deterred cutter formation (fig. 
29). At the Greenwich Mines, some success has been had 
using this technique. Cribs and posts should be installed 
shortly after mining, and the support should be placed in 
such a way as not to yield significantly (e.g., a minimum of 
cap pieces). 

With respect to conventional bolting practices, cutter 
failure fi-equently propagates to just above the anchor 
horizon, resulting in massive roof failure. Changes in bolt 
length most often result in only a change in the height to 
which cutter failure propagates. However, for cases of 
cutter failure that are not severe, some operators have 
found a combination of bolt lengths across an entry to be 
successful (e.g., bolts next to ribline are shorter than bolts 
in center of entry). Another remedy has been to use 
point-anchor-resin, tensioned, rebar-type bolts and to 
ensure that these bolts are uniformly tensioned, upon 
installation, throughout the entry. 



MINE DESIGN CHANGES 

Mine design changes, as control measures, frequently 
take advantage of the elemental factors that cause cutter 
roof failure. In the following examples, rock property data 
and knowledge of the in situ stress state are valuable in 
assessing the probability of success and developing an 
implementation plan for the control measure. Obtaining 
accurate measurements of the in situ stress state and 
meaningful rock property values is difficult, but often 
necessary. In the literature are several cases where mine 
officials used mine design changes to control cutter roof 



failure without the aid of rock mechanics data. However, 
since a great deal of time and expense is invested in 
making mine design changes, the benefits of having these 
data are obvious. 

Sacrifice Entries^ 

Sacrifice entries have been used as a method of 
ground control since the 18th century, and dating the 
common use of sacrifice entries in the United States at 
approximately 1935, Roberts (42) referred to this method 
as the use of caving chambers and described it as follows: 

In essence, the caving chamber is an auxiliary 
road, driven parallel to the main roads, and kept 
slightly in advance of them. At short periodic 
intervals all supports are withdrawn from the 
caving chamber, which is allowed to collapse, and 
as result of the falls of roof in this chamber the 
roof in the neighboring roads remains solid. 

The premise for using sacrifice entries is that" 
extremely high horizontal in situ stresses exist (or that 
measurements have shown that high in situ stresses exist) 
that are thought to be the primary cause of roof failure at 
the site, and further, that by initiating failure in an entry 
ahead of and parallel to future adjacent entries, the in situ 
stress can be relieved to manageable levels, allowing 
adjacent entries to advance without incident. Nicholls (38) 
has discussed the use of this method in conjunction with 
the problem of cutter roof failure in Australia and 
reported limited success. 

Figure 30 illustrates the use of contemporary caving 
chambers in three-entry gateroads. Roof rock is mined in a 



^he author thanks Nicolas P. Kripakov, mining engineer, Denver 
Research Center, Bureau of Mines, Denver, CO, for providing the results of 
his finite-element analysis of the arch-sacrifice-entry method and for 
relating his experience with the subject. 



» 



Clastic r^V 

dike '-'^ 




-Cribbing ^^ 



Entry 







20 



Scale, ft 



Figure 29. — Plan view of mine entry showing placement of 
cribbing adjacent to clastic dike to deter cutter failure 
formation. 



o-h =1,900 PS 



23 




Center -lo-center 
dimension 



Center- to- center 
dimension, ft 


Reduction in sheor stress 
at entry corner, pet 


Inside (1) 


Outside (0) 


50 
40 
30 


5 2 

1 1 3 
24 6 


3 5 
5 7 

10,1 



A, CENTER ENTRY USED FOR STRESS RELIEF 



I \ I 



C 



I i o^i^ 700 psi 



Center -to -center 
dimension, ft 


Reduction in stiear stress 
at entry corner (C), pet 


50 
40 
30 


82 

156 
28 3 



crh=l900psi Center -to -center 

' dimension 

e, OUTER 2 ENTRIES USED FOR STRESS RELIEF 

Figure 30. — Contemporary versions of caving chambers for use in three-entry gateroad configurations. (Courtesy N. P. Kripakov) 



rough arch outline to a height above the coalbed in either 
the center entry (fig. 30A) or the two other entries (fig. 
SOB), with only steel arches and steel lagging as support. 
The entry (or entries) mined with the arches is driven 
approximately 100 ft in advance of the other entries. 
Approximately 18 in of space is left between the steel 
arches and the newly exposed roof surface (an arbitrary 
amount of space leaving sufficient room for expansion), 
thus allowing the roof to cave onto the arches and relieve 
in situ stresses. Upon abandonment of the gateroad, it 
may be possible to retrieve the arches and lagging for 
future use. 

Kripakov conducted the finite-element analysis of the 
two different scenarios using the stress values and rock 
properties used to generate the results shown in table 1. 
The results of the analysis are shown in figure 30, 
demonstrating that reduction of pillar sizes increases the 
effectiveness of the caving, as does the use of two arched 
entries as opposed to one. Since Kripakov's finite-element 
code generally treats rock as a continuum, the actual 
reduction in shear stress may be much greater in the 
actual mine environment because of shear displacement 
along bedding planes. The basic principle in reducing the 
shear stress occurring at an entry corner is reducing the 
difference in the principal stresses. While the magnitude 
of the stresses is important, the difference in magnitudes 
can control failure. 

Although sacrifice entries are not in wide use today, 
their success in the past suggests some possibilities for 
future use. From the differences between the old caving 
chamber, as explained by Roberts, and the contemporary 
sacrifice entry, which requires special equipment and 
arches, it is obvious that companies must go to great effort 
to incorporate this method as a regular practice. 

Pillar Softening And Yield Pillars 

Pillar softening is a technique developed by Wang 
|-?7i for reducing the magnitude of stress concentrations in 
the corners of entries. The concept is based on coal pillar 



elasticity versus roof rock elasticity, discussed in the 
section "Rock Mass Characteristics," and the effect of this 
ratio on the magnitude of stresses in the corners of the 
entry (fig. 14). By reducing the elasticity of the pillars to 
some distance away from the entry, the stress concentra- 
tion in the corners of the entry are effectively redis- 
tributed. Using finite-element analysis, Wang found that 
by drilling 6-in-diam holes into the pillars and face as an 
entry was advanced, the elasticity of the pillar near the 
rib and the stress in the corners of the entry could be 
reduced. 




Softenedl 
zone I 



KEY 
■ First advance 
Second advance 



Figure 31. — Plan for placement of auger holes for pillar- 
softening concept. [Adapted from Maxwell (34)\ 



24 




5 

Scale, ft 

Figure 32.— Cross-sectional view of rib-slotting method. 
[Adapted from Kripakov (27)] 

The pillar-softening concept was tested in Mine 32 of 
the Bethlehem Mine Corp. (34) near Ebensburg, PA, an 
area of the Appalachian Basin plagued by the problem of 
cutter roof failure. Figure 31 illustrates the pattern of 
holes used during the testing. An attempt to measure the 
in situ horizontal stress was made, with inconclusive 
results, and other measurements were made of pillar 
stresses, material properties, roof strain, convergence, and 
tilt. The results of the tests showed a reduction in stress in 
the corners of the entry; however, there was no indication 
that roof stability improved as a result of the softening. 

Kripakov (27) further investigated Wang's method 
and found that an optimum location for softening holes 
was at the roof-coal interface, which resulted in an 
effective decrease in stress concentration at the comers of 
10 pet (table 1). Since the original pillar-softening test at 
Mine 32 did not show a significant improvement in roof 
conditions, Kripakov took the method a step further and 
introduced the concept of rib slotting (fig. 32). Through 
finite-element analysis, he found a 40-pct reduction in 
stress concentrations in the corners of the entry when 
horizontal 3-in-thick slots were created at the roof-coal 
interface (table 1). The method has not been exhaustively 
tested, and a means of efficiently cutting slots in the ribs 
has yet to be presented. However, the initial analysis of 
the method suggests it may be worthy of additional 
testing. A great potential for limited use of this concept 
exists for special cases, such as cutter failure in the outer 
entries of a multiple-entry development. The pillar- 
softening method may also be useful when a face area is to 
be left idle for several days. In situations like this, the face 
area essentially becomes an outer entry in a multiple- 
entry scenario. Pillar softening in the area may deter 
failure until mining is resumed and additional support 
can be installed. 

Another method of reducing the elasticity of a coal 
pillar, the yield pillar design, has recently reemerged in 
the coal mining industry as a viable ground control 
method. While the method is being used only ex- 
perimentally for controlling classic cutter roof failure, at 
least three mines are using yield pillars for other ground 
control problems. The basic concept is that pillars are 
small enougb so they intentionally yield and transfer the 
majority of roof loads to the abutments. The result is a 
reduction of overall roof and floor stresses (49). Kripakov 
(27) suggested yield pillars as a possible solution for 



Slot - 




A PLAN VIEW 



10 r 

h 

10 
Scale, ft 



Slot holes 




S, CROSS SECTION VIEW 

Figure 33. — Placement of holes for roof-slotting method. 
[Adapted from Maxwell (34)] 

controlling cutter roof failure at the Kitt Min§, although 
in-mine verification was never conducted. 

Roof Slotting 

In conjunction with the testing of the pillar-softening 
method at Mine 32 of the Bethlehem Mine Corp., a second 
method for controlling cutter roof failure was tested. 



25 



which consisted of drilling a series of holes adjacent to the 
entry to form a vertical slot in the roof rock above the 
pillar (fig. 33). Entries treated by slotting prior to 
development showed a marked improvement in roof 
conditions over adjacent entries, and instrumentation 
revealed that the slots did provide for relief of horizontal 
stress. Unfortunately, because of the inability to use this 
method efficiently during production, it is not very 
practical. But the initial indications of success suggest 
that future efforts aimed at developing a means for using 
the method during production would be worthwhile. 

Entry Reorientation 

The practice of reorienting entries, in an attempt to 
eliminate roof falls that occur in entries of a particular 
orientation, has been a successful control method when 
applied to problem ground conditions caused by a biaxial 
horizontal stress field. The theoretical premise for the 
success of this method lies in the relation of horizontal to 
vertical stress, discussed under the heading of "Stress 
Environment." If the horizontal stress is strongly biaxial, 
the entries oriented perpendicular to the maximum 
principal horizontal stress are under the greatest in- 
fluence of the stress. By reorienting mine headings so that 
both entries are perpendicular to the same least stress 
value {fig. 34), the influence of the biaxial stress field can 
be offset. However, in many cases, reorientation to obtain 
least stress values is not an adequate solution. For these 
cases, a solution can be found by orienting the main 
headings parallel with the maximum principal horizontal 
in situ stress and staggering pillars, thus isolating the 
occurrence of roof failure to crosscuts. In addition, Aggson 
(3) suggests a reduction in the width of crosscut entries to 
further reduce the probability of failure. 

In the Kitt Mine, only a 22-pct difference in 
magnitude was found between the minor and major 
principal horizontal stresses. In this case, reorienting 
entry headings to an optimum angle from the principal 
horizontal stresses would not decrease stress perpendicu- 
lar to the entries enough to create entry stability. In fact, 
the greatest reduction that could be realized by reorienta- 
tion of the entries would be only an 11-pct reduction of the 
major principal horizontal stress. 

Entry reorientation has been used to control cutter 
roof failure (and other types of failure) caused by a highly 
biaxial, horizontal stress field in cases where the stress 
field was of local occurrence and in other cases where it 
was of regional occurrence. Local cases have usually been 
associated with sedimentary or compactional features, 
such as sandstone channels or rolls. Connelly ilO) refers to 
the use of entry reorientation in Australia as a successful 
method for offsetting the influence of "stone rolls." In the 
case he cites, headings were reoriented to intersect the 
rolls at an oblique angle, resulting in an immediate 
improvement in roof conditions. In the United States, 
similar conditions have been found, as in the West 
Virginia mine shown in figure 21. Reorientation was also 
used at this mine in an attempt to offset the influence of 
the structure, and no further failure ensued. However, in 
cases such as this, in-mine verification of a local biaxial 
stress field has not been established and reorientation has 
not been verified, through instrumentation, as the cause 
of improvement. The Pennsylvania mine shown in figure 
11 displays similar characteristics; in the section oriented 
45 from the headings of the rest of the mine there is no 



A 

/ 
/ 
/ 
/ 

/ 
/ 
/ 




^ // / /A 

/ 
/ 
/ 

/ 



/ 650 psi 



A, 



ORIENTED PERPENDICULAR TO 
MAJOR PRINCIPAL STRESS 



650 
psi 



1,300 
psi 




975 psi 
as calculated 
from av stress 



B, ORIENTED 45* FROM MAJOR 
PRINCIPAL STRESS 

Figure 34. — Apparent values of horizontal in situ stress for 
the two extremes of entry orientation versus orientation of 
actual principal in situ stress. 

roof failure. While these two cases are apparently 
successful, in many other cases no deterrence of failure 
was realized. 

Some operators have reported on the implementation 
and success of entry reorientation and its use in 
controlling cutter roof failure associated with a regionally 
high biaxial stress field. In some cases (7-8) in situ stress 
measurements were not taken to quantify the stress field 
but rather the decision to implement changes was made 
based on in-mine observations of roof failure. In another 



26 



case (29), attempts were made to quantify the stress field 
before implementing successful reorientation changes, 
although the results were inconclusive based on the fact 
that the deepest overcoring measurement was taken only 
6 ft from the mine opening. 

Although defining the stress environment and other 
factors of roof failure propagation is important, reasonable 
engineering decisions can be made based on keen 
observations, as was done in the cases cited above. The 
value of having as much information as possible is 
obvious, however, in light of the magnitude of the changes 
made. 

INDIRECT CONTROL MEASURES 

Other ideas about controlling cutter roof failure 
include the same kind of reasoning that suggests planing 



around stream valley areas that may be susceptible to 
failure. This type of reasoning leads to the conclusion that 
specific mining methods may actually reduce the threat of 
encountering severe cutter roof failure. Many mines 
currently experiencing cutter roof failure are employing 
the room-and-pillar mining method, which exposes a 
tremendous amount of roof area that must be supported 
over long periods of time. In regions where the threat of 
cutter failure is such that control may be impractical or 
impossible, the possibility of using the longwall mining 
method should be assessed. For a given mined area, this 
method exposes less roof to long-term support needs; 
therefore, there is less potential for having to deal with 
the problem. Additionally, gateroad systems lend them- 
selves to short-term innovative kinds of control measures, 
such as the sacrifice entry concept previously explained. 



CONCLUSIONS AND RECOMMENDATIONS 



This report outlines the most commonly cited theories 
on the formation of cutter roof failure and the most 
commonly suggested methods for controlling its occur- 
rence. No unique solution exists for controlling or avoiding 
cutter roof failure. Some operators have had limited 
success in controlling failure propagation by systematical- 
ly analyzing the patterns of failure in their mines and 
then, through a process of elimination, selecting a control 
method. The limitations on their success may stem from 
the fact that although theories on cutter failure exist, few 
have been verified through in-mine experimentation. 
Likewise, when control measures have been implemented, 
inadequate monitoring has resulted in an inability to 
determine the overall effect of the measure on roof 
stability. Continued research is needed by mine operators, 
the Bureau, and other research organizations, so that 
appropriate modifications can be made to existing theories 
and control methods to increase their rate of success. 

In addition to using the decision process diagram 
illustrated in figure 24 (for the basic determination of 



probable causes of failure and subsequently for the 
selection of a control measure), mine operators are 
encouraged to consider mining methods that reduce the 
amount of exposed roof that must be supported for long 
periods of time. Longwall mining meets this criterion in a 
manner that provides fiexibility for the employment of 
innovative control methods such as yield pillars, re- 
orientation of entries, and sacrifice entries. In localized 
areas of a relatively high probability of cutter roof failure, 
the basic premining plan may be altered to take into 
account these areas and either use them for barrier pillar 
areas or only mine them upon retreat. 

Until future advances are made in understanding the 
cutter failure phenomenon, the present state of the art 
does provide options for mining in areas where this type of 
failure occurs frequently. However, it remains the 
responsibility of individual mine operators to weigh the 
cost of resupporting failed areas and the threat of injury to 
miners against the investment made to select and 
implement control methods. 



REFERENCES 



1. Agapito, J. F. T., J. R. Aggson, S. J. Mitchell, M. P. Hardy, 
and W. N. Hoskins. Study of Ground Control Problems in Coal 
Mines With High Horizontal Stresses. Paper in Proceedings of 
Twenty First Symposium on Rock Mechanics: A State of The Art. 
Univ. MO, Rolla, MO, 1980, pp. 820-828. 

2. Aggson, J. R. Coal Mine Floor Heave in the Beckley 
Coalbed, An Analysis. BuMines RI 8274, 1978, 32 pp. 

3. How To Plan Ground Control. Coal Min. & 

Process., v. 16, No. 12, 1979, pp. 70-73. 

4. Stress-Induced Failures in Mine Roof. BuMines 

RI 8338, 1979, 16 pp. 

5. Barish, K. Truss Bolting On-Cycle in Jane Mine Lower 
Freeport Seam. Paper in Proceedings, Fourth Conference on 
Ground Control in Mining. WV Univ., Morgantown, WV, 1985, 
pp. 1-10. 

6. Bieniawski, Z. T. Rock Mechanics Design in Mining and 
Tunneling. A. A. Balkema, 1984, 272 pp. 

7. Blevins, C. T. Coping With High Lateral Stresses in an 
Underground Illinois Coal Mine. Soc. Min. Eng. AIME preprint 
82-156, 1982, 6 pp. 



8. Blevins, C. T., and D. Dopp. Ground Control Experiences in 
a High Horizontal Stress Field at Inland Steel Coal Mine No. 2. 
Paper in Proceedings, Fourth Conference on Ground Control in 
Mining. WV Univ., Morgantown, WV, 1985, pp. 227-233. 

9. Chase, F. E. Clay Veins: Their Physical Characteristics, 
Prediction, and Support. Paper in Proceedings, Fourth Confer- 
ence on Ground Control in Mining. WV Univ., Morgantown, WV, 
1985, pp. 212-219. 

10. Connelly, M. A. The Uses of Geologic Structural Analyses 
in Predicting Roof Conditions in Coal Mining. Paper in 
Proceedings of the Symposium on Stress and Failure Around 
Underground Openings. Univ. Sydney, Sydney, N.S.W., Austra- 
lia, paper 13, 1967, pp. 1-2. 

11. Enever, J. R., and J. McKay. Stress Measurements at 
Nattai North Colliery and Their Interpretation in Terms of 
Sedimentological and Topographic Features. CSIRO, Div. Ap- 
plied Geomechanics, Mount Waverley, Victoria, Australia, Rep. 
29, 1980, 12 pp. 

12. Enever, J. R., J. Shepherd, and J. Huntington. An Initial 
Mathematical Relationship Between Underground Working 



27 



Conditions and Overlying Surface Topography With Reference to 
the Western Coalfields, N.S.W. CSIRO, Div. Mineral Physics, 
Mount Waverley, Victoria, Australia. Rep. 5, 1978, 6 pp. 

13. Engelder, T. Is There a Genetic Relationship Between 
Selected Regional Joints and Contemporary Stress Within the 
Lithosphere of North America? Tectonics, v. 1, No. 2, 1982, pp. 
161-177. 

14. Ferguson, H. F. Geological Observations and Geotechnical 
Effects of Valley Stress Relief in the Allegheny Plateaus. Pres. at 
Annu. ASCE Meet., Los Angeles, CA, 1974, 31 pp.; available from 
J. L. Hill III, BuMines, Pittsburgh, PA. 

15. Valley Stress Release in the Allegheny 

Plateau. Bull. Assoc. Eng. Geol., v. 4, No. 1, 1967, pp. 63-69. 

16. Haycocks, C, M. Karmis, E. Barko, J. Chaman, S. Hudock, 
B. Ehgartner, and S. Webster. Ground Control Mechanics in 
Multiple-Seam Mining (grants G1105050 and G1115511, VPI), 
BuMines OFR 7-84. 

17. Haycocks, C, M. Karmis, and B. Ehgartner. Multiple 
Seam Mine Design. Paper in State-of-the-Art of Ground Control 
in Longwall Mining and Mining Subsidence. Soc. Min. Eng. 
AIME, 1982, pp. 59-65. 

18. Haycocks, C, M. Karmis, and E. Topuz. Optimizing 
Productive Potential in Multi-Seam Underground Coal Mining. 
Paper in Coal Conference & Expo VI. McGraw-Hill, 1981, 
pp.151-164. 

19. Hill, J. L. Ill, and E. R. Bauer. An Investigation of The 
Causes of Cutter Roof Failure in a Central Pennsylvania Coal 
Mine: A Case Study. Paper in Rock Mechanics in Productivity 
and Protection, Twenty-Fifth Symposium on Rock Mechanics. 
Soc. Min. Eng, AIME, 1984, pp. 603-614. 

20. Hoek, E., and E. T. Brown. Underground Excavations in 
Rock. Inst. Min. and Metall., London, 1980, pp. 110-243. 

21. Hylbert, D. K. The Classification, Evaluation and Projec- 
tion of Coal Mine Roof Rocks in Advance of Mining. Min. Eng. 
(Littleton, CO), v. 30, 1978, pp. 1667-1676. 

22. Developing Geological Structural Criteria for 

Predicting Unstable Mine Roof Rocks (contract HO 133018, 
Appalachian Coal Min. Inst., Moorhead State Univ.). BuMines 
OFR 9-78, 1977, 249 pp.; NTIS PB 276-735/AS. 

23. lannacchione, A. T., J. T. Popp, and J. A. Rulli. The 
Occurrence and Characterization of Geologic Anomalies and 
Cutter Roof Failure: Their Affect on Gateroad Stability. Paper in 
Stability in Underground Mining 11. Soc. Min. Eng. AIME, 1984, 
pp. 428-445. 

24. Jeffrey, R. G. Jr., and J. J. K. Daemen. Analysis of 
Rockbolt Reinforcement of Layered Rock Using Beam Equations. 
Paper in Proceedings of the International Symposium on Rock 
Bolting, A. A. Balkema, 1983, pp. 173-185. 

25. Kent, H. B. Geologic Causes and Possible Preventions of 
Roof Fall in Room-And-Pillar Coal Mines. PA Bur. Topogr. and 
Geol. Surv., Inf Circ. 75, 1974, 17 pp. 

26. Kertis, C. A, Reducing Hazards in Underground Coal 
Mines Through the Recognition and Delineation of Coalbed 
Discontinuities Caused by Ancient Channel Processes, BuMines 
RI 8987, 1985, 23 pp. 

27. Kripakov, N. P. Alternatives for Controlling Cutter Roof in 
Coal Mines. Paper in Proceedings, Second Conference on Ground 
Control in Mining. WV Univ., Morgantown, WY., 1982, pp, 
142-151. 

28. Lang, T. A, Theory and Practice of Rock Bolting, Trans, 
Soc, Min. Eng. AIME, v, 223, 1962, pp, 333-348. 

29. Lizak, J, B. and J. E. Semborski. Horizontal Stresses and 
Their Impact on Roof Stability at the Nelms No, 2 Mine. Pres. at 



4th Conf on Ground Control in Mining, WV Univ., Morgantown, 
WV, July 22-24, 1985, 7 pp.; available from J. L. Hill III, 
SuMines, Pittsburgh, PA. 

30. Mallicoat, W. R. Truss Bolting With Point Resin Anchor- 
age. Min. Congr. J., v. 64, June 1978, pp. 47-50. 

31. Mangelsdorf, C. P. Current Trends in Roof Truss Hard- 
ware. Paper in Proceedings, Second Conference on Ground 
Control in Mining. WV Univ., Morgantown, WV, 1982, pp. 
108-112. 

32. Evaluation of Roof Trusses, Phase I (contract 

GO166088, Univ. Pittsburgh). BuMines OFR 56-82, 1979, 111 
pp.; NTIS PB 82-209768. 

33. The Role of Friction in Roof Truss Behavior. 

Trans. Soc. Min. Eng. AIME, v. 268, 1980, pp, 1869-1879, 

34. Maxwell, B., G. Zink, and F, D, Wang, Improving Coal 
Mine Roof Stability by Pillar Softening (contract GO133063, CO 
Sch. Mines), BuMines OFR 7-78, 1977, 105 pp,; NTIS PB 276-474. 

35. Moebs, N. N. Roof Rock Structures and Related Roof 
Support Problems in the Pittsburgh Coalbed of Southwestern 
Pennsylvania. BuMines RI 8230, 1977, 30 pp. 

36. Moebs, N. N., and R. M. Stateham, The Diagnosis and 
Reduction of Mine Roof Failure, Coal Min., v. 22, 1985, pt. 1, No. 
2, 1985, pp. 52-55; pt. 2, No, 3, 1985, pp, 42-48. 

37. Geologic Factors in Coal Mine Roof Stability — 

A Progress Report. BuMines IC 8976, 1984, 27 pp. 

38. Nicholls, B. Pillar Extraction on the Advance at Oakdale 
Colliery. Paper in Proceedings of the First International 
Conference on Stability in Coal Mining. Miller Freeman Publ., 
1978, pp. 182-196. 

39. Obert, L., W. I. Duvall, and R. H. Merrill. Design of 
Underground Openings in Competent Rock. BuMines B 587, 
1960, 36 pp. 

40. Peng, S. S., and U. Chandra. Getting The Most From 
Multiple Seam Reserves. Coal Min. & Process., v. 17, No. 11, 
1980, pp. 78-84. 

41. Phillips, D. W. American Coal Mining. Colliery Guardian, 
V. 175, No. 4523, 1947, pp. 37-43. 

42. Roberts, A. A Review of the Problem of Strata Control in 
Bord and Pillar Working. Colliery Guardian, v. 170, No. 4404, 
1945, pp. 663-668. 

43. Roley, R. W. "Pressure-Cutting": A Phenomenon of 
Coal-Mine Roof Failures. Mechanization, v. 12, No. 12, 1948, pp. 
69-74. 

44. Su, W. H., and S. S. Peng. Cutter Roof and Its Causes. Soc. 
Min. Eng. AIME preprint 85-131, 1985, 5 pp. 

45. Thomas, E. Conventional Timbering Versus Suspension 
Supports. BuMines B, 489, 1950, pp, 175-181, 

46. Thomas, E., A. J. Barry, and A. Metcalf Suspension 
Support Progress Report. BuMines IC 7533, 1949, 13 pp. 

47. Wang, F, D., D. M, Ropchan, and M, C, Sun. Proposed 
Technique for Improving Coal-Mine Roof Stability by Pillar 
Softening. Trans. Soc. Min. Eng, AIME, v, 255, 1974, pp, 59-63. 

48. Structural Analysis of a Coal Mine Opening in 

Elastic Multilayered Material. BuMines RI 7845, 1974, 36 pp, 

49, Wilson, A, H. The Effect of Yield Zones on the Control of 
Ground. Paper in Sixth International Strata Control Conference. 
Natl. Coal Board, Burton-on-Trent, Staffordshire, England, Sept. 
1977, pp. 52-93. 

50. Worotnicki, G, Effect of Topography on Ground Stresses. 
Presented at Rock Mechanics Symp., Univ. Sydney, Sydney, 
N.S.W., Australia, Feb. 19-20, 1969, 12 pp.; available from J. L. 
Hill III, BuMines, Pittsburgh, PA. 



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